2.5. The GHCi Debugger

GHCi contains a simple imperative-style debugger in which you can
stop a running computation in order to examine the values of
variables. The debugger is integrated into GHCi, and is turned on by
default: no flags are required to enable the debugging
facilities. There is one major restriction: breakpoints and
single-stepping are only available in interpreted modules;
compiled code is invisible to the debugger[5].

The debugger provides the following:

The ability to set a breakpoint on a
function definition or expression in the program. When the function
is called, or the expression evaluated, GHCi suspends
execution and returns to the prompt, where you can inspect the
values of local variables before continuing with the
execution.

Execution can be single-stepped: the
evaluator will suspend execution approximately after every
reduction, allowing local variables to be inspected. This is
equivalent to setting a breakpoint at every point in the
program.

Execution can take place in tracing
mode, in which the evaluator remembers each
evaluation step as it happens, but doesn't suspend execution until
an actual breakpoint is reached. When this happens, the history of
evaluation steps can be inspected.

Exceptions (e.g. pattern matching failure and
error) can be treated as breakpoints, to help
locate the source of an exception in the program.

There is currently no support for obtaining a “stack
trace”, but the tracing and history features provide a
useful second-best, which will often be enough to establish the
context of an error. For instance, it is possible to break
automatically when an exception is thrown, even if it is thrown
from within compiled code (see Section 2.5.6, “Debugging exceptions”).

Now, let's set a breakpoint on the right-hand-side of the second
equation of qsort:

*Main> :break 2
Breakpoint 0 activated at qsort.hs:2:15-46
*Main>

The command :break 2 sets a breakpoint on line
2 of the most recently-loaded module, in this case
qsort.hs. Specifically, it picks the
leftmost complete subexpression on that line on which to set the
breakpoint, which in this case is the expression
(qsort left ++ [a] ++ qsort right).

Now, we run the program:

*Main> main
Stopped at qsort.hs:2:15-46
_result :: [a]
a :: a
left :: [a]
right :: [a]
[qsort.hs:2:15-46] *Main>

Execution has stopped at the breakpoint. The prompt has changed to
indicate that we are currently stopped at a breakpoint, and the location:
[qsort.hs:2:15-46]. To further clarify the
location, we can use the :list command:

The :list command lists the source code around
the current breakpoint. If your output device supports it, then GHCi
will highlight the active subexpression in bold.

GHCi has provided bindings for the free variables[6] of the expression
on which the
breakpoint was placed (a, left,
right), and additionally a binding for the result of
the expression (_result). These variables are just
like other variables that you might define in GHCi; you
can use them in expressions that you type at the prompt, you can ask
for their types with :type, and so on. There is one
important difference though: these variables may only have partial
types. For example, if we try to display the value of
left:

[qsort.hs:2:15-46] *Main> left
<interactive>:1:0:
Ambiguous type variable `a' in the constraint:
`Show a' arising from a use of `print' at <interactive>:1:0-3
Cannot resolve unknown runtime types: a
Use :print or :force to determine these types

This is because qsort is a polymorphic function,
and because GHCi does not carry type information at runtime, it cannot
determine the runtime types of free variables that involve type
variables. Hence, when you ask to display left at
the prompt, GHCi can't figure out which instance of
Show to use, so it emits the type error above.

Fortunately, the debugger includes a generic printing command,
:print, which can inspect the actual runtime value of a
variable and attempt to reconstruct its type. If we try it on
left:

This isn't particularly enlightening. What happened is that
left is bound to an unevaluated computation (a
suspension, or thunk), and
:print does not force any evaluation. The idea is
that :print can be used to inspect values at a
breakpoint without any unfortunate side effects. It won't force any
evaluation, which could cause the program to give a different answer
than it would normally, and hence it won't cause any exceptions to be
raised, infinite loops, or further breakpoints to be triggered (see
Section 2.5.3, “Nested breakpoints”).
Rather than forcing thunks, :print
binds each thunk to a fresh variable beginning with an
underscore, in this case
_t1.

The flag -fprint-evld-with-show instructs
:print to reuse
available Show instances when possible. This happens
only when the contents of the variable being inspected
are completely evaluated.

If we aren't concerned about preserving the evaluatedness of a
variable, we can use :force instead of
:print. The :force command
behaves exactly like :print, except that it forces
the evaluation of any thunks it encounters:

[qsort.hs:2:15-46] *Main> :force left
left = [4,0,3,1]

Now, since :force has inspected the runtime
value of left, it has reconstructed its type. We
can see the results of this type reconstruction:

We evaluated only the _t1 thunk, revealing the
head of the list, and the tail is another thunk now bound to
_t2. The seq function is a
little inconvenient to use here, so you might want to use
:def to make a nicer interface (left as an exercise
for the reader!).

The execution continued at the point it previously stopped, and has
now stopped at the breakpoint for a second time.

2.5.1.1. Setting breakpoints

Breakpoints can be set in various ways. Perhaps the easiest way to
set a breakpoint is to name a top-level function:

:break identifier

Where identifier names any top-level
function in an interpreted module currently loaded into GHCi (qualified
names may be used). The breakpoint will be set on the body of the
function, when it is fully applied but before any pattern matching has
taken place.

When a breakpoint is set on a particular line, GHCi sets the
breakpoint on the
leftmost subexpression that begins and ends on that line. If two
complete subexpressions start at the same
column, the longest one is picked. If there is no complete
subexpression on the line, then the leftmost expression starting on
the line is picked, and failing that the rightmost expression that
partially or completely covers the line.

When a breakpoint is set on a particular line and column, GHCi
picks the smallest subexpression that encloses that location on which
to set the breakpoint. Note: GHC considers the TAB character to have a
width of 1, wherever it occurs; in other words it counts
characters, rather than columns. This matches what some editors do,
and doesn't match others. The best advice is to avoid tab
characters in your source code altogether (see
-fwarn-tabs in Section 4.8, “Warnings and sanity-checking”).

If the module is omitted, then the most recently-loaded module is
used.

Not all subexpressions are potential breakpoint locations. Single
variables are typically not considered to be breakpoint locations
(unless the variable is the right-hand-side of a function definition,
lambda, or case alternative). The rule of thumb is that all redexes
are breakpoint locations, together with the bodies of functions,
lambdas, case alternatives and binding statements. There is normally
no breakpoint on a let expression, but there will always be a
breakpoint on its body, because we are usually interested in inspecting
the values of the variables bound by the let.

2.5.1.2. Listing and deleting breakpoints

The list of breakpoints currently enabled can be displayed using
:show breaks:

To delete a breakpoint, use the :delete
command with the number given in the output from :show breaks:

*Main> :delete 0
*Main> :show breaks
[1] Main qsort.hs:2:15-46

To delete all breakpoints at once, use :delete *.

2.5.2. Single-stepping

Single-stepping is a great way to visualise the execution of your
program, and it is also a useful tool for identifying the source of a
bug. GHCi offers two variants of stepping. Use
:step to enable all the
breakpoints in the program, and execute until the next breakpoint is
reached. Use :steplocal to limit the set
of enabled breakpoints to those in the current top level function.
Similarly, use :stepmodule to single step only on
breakpoints contained in the current module.
For example:

*Main> :step main
Stopped at qsort.hs:5:7-47
_result :: IO ()

The command :step
expr begins the evaluation of
expr in single-stepping mode. If
expr is omitted, then it single-steps from
the current breakpoint. :stepover
works similarly.

The :list command is particularly useful when
single-stepping, to see where you currently are:

2.5.3. Nested breakpoints

When GHCi is stopped at a breakpoint, and an expression entered at
the prompt triggers a
second breakpoint, the new breakpoint becomes the “current”
one, and the old one is saved on a stack. An arbitrary number of
breakpoint contexts can be built up in this way. For example:

While stopped at the breakpoint on line 2 that we set earlier, we
started a new evaluation with :step qsort [1,3].
This new evaluation stopped after one step (at the definition of
qsort). The prompt has changed, now prefixed with
..., to indicate that there are saved breakpoints
beyond the current one. To see the stack of contexts, use
:show context:

2.5.4. The _result variable

When stopped at a breakpoint or single-step, GHCi binds the
variable _result to the value of the currently
active expression. The value of _result is
presumably not available yet, because we stopped its evaluation, but it
can be forced: if the type is known and showable, then just entering
_result at the prompt will show it. However,
there's one caveat to doing this: evaluating _result
will be likely to trigger further breakpoints, starting with the
breakpoint we are currently stopped at (if we stopped at a real
breakpoint, rather than due to :step). So it will
probably be necessary to issue a :continue
immediately when evaluating _result. Alternatively,
you can use :force which ignores breakpoints.

2.5.5. Tracing and history

A question that we often want to ask when debugging a program is
“how did I get here?”. Traditional imperative debuggers
usually provide some kind of stack-tracing feature that lets you see
the stack of active function calls (sometimes called the “lexical
call stack”), describing a path through the code
to the current location. Unfortunately this is hard to provide in
Haskell, because execution proceeds on a demand-driven basis, rather
than a depth-first basis as in strict languages. The
“stack“ in GHC's execution engine bears little
resemblance to the lexical call stack. Ideally GHCi would maintain a
separate lexical call stack in addition to the dynamic call stack, and
in fact this is exactly
what our profiling system does (Chapter 5, Profiling), and what
some other Haskell debuggers do. For the time being, however, GHCi
doesn't maintain a lexical call stack (there are some technical
challenges to be overcome). Instead, we provide a way to backtrack from a
breakpoint to previous evaluation steps: essentially this is like
single-stepping backwards, and should in many cases provide enough
information to answer the “how did I get here?”
question.

To use tracing, evaluate an expression with the
:trace command. For example, if we set a breakpoint
on the base case of qsort:

Note that the local variables at each step in the history have been
preserved, and can be examined as usual. Also note that the prompt has
changed to indicate that we're currently examining the first step in
the history: -1. The command
:forward can be used to traverse forward in the
history.

The :trace command can be used with or without
an expression. When used without an expression, tracing begins from
the current breakpoint, just like :step.

The history is only available when
using :trace; the reason for this is we found that
logging each breakpoint in the history cuts performance by a factor of
2 or more. GHCi remembers the last 50 steps in the history (perhaps in
the future we'll make this configurable).

2.5.6. Debugging exceptions

Another common question that comes up when debugging is
“where did this exception come from?”. Exceptions such as
those raised by error or head []
have no context information attached to them. Finding which
particular call to head in your program resulted in
the error can be a painstaking process, usually involving
Debug.Trace.trace, or compiling with
profiling and using Debug.Trace.traceStack
or +RTS -xc (see Section 5.3, “Time and allocation profiling”).

The GHCi debugger offers a way to hopefully shed some light on
these errors quickly and without modifying or recompiling the source
code. One way would be to set a breakpoint on the location in the
source code that throws the exception, and then use
:trace and :history to establish
the context. However, head is in a library and
we can't set a breakpoint on it directly. For this reason, GHCi
provides the flags -fbreak-on-exception which causes
the evaluator to stop when an exception is thrown, and
-fbreak-on-error, which works similarly but stops only on
uncaught exceptions. When stopping at an exception, GHCi will act
just as it does when a breakpoint is hit, with the deviation that it
will not show you any source code location. Due to this, these
commands are only really useful in conjunction with
:trace, in order to log the steps leading up to the
exception. For example:

Breaking on exceptions is particularly useful for finding out what
your program was doing when it was in an infinite loop. Just hit
Control-C, and examine the history to find out what was going
on.

2.5.7. Example: inspecting functions

It is possible to use the debugger to examine function values.
When we are at a breakpoint and a function is in scope, the debugger
cannot show
you the source code for it; however, it is possible to get some
information by applying it to some arguments and observing the result.

The process is slightly complicated when the binding is polymorphic.
We show the process by means of an example.
To keep things simple, we will use the well known map function:

GHCi tells us that, among other bindings, f is in scope.
However, its type is not fully known yet,
and thus it is not possible to apply it to any
arguments. Nevertheless, observe that the type of its first argument is the
same as the type of x, and its result type is shared
with _result.

As we demonstrated earlier (Section 2.5.1, “Breakpoints and inspecting variables”), the
debugger has some intelligence built-in to update the type of
f whenever the types of x or
_result are discovered. So what we do in this
scenario is
force x a bit, in order to recover both its type
and the argument part of f.

*Main> seq x ()
*Main> :print x
x = 1

We can check now that as expected, the type of x
has been reconstructed, and with it the
type of f has been too:

*Main> :t x
x :: Integer
*Main> :t f
f :: Integer -> b

From here, we can apply f to any argument of type Integer and observe
the results.

In the first application of f, we had to do
some more type reconstruction
in order to recover the result type of f.
But after that, we are free to use
f normally.

2.5.8. Limitations

When stopped at a breakpoint, if you try to evaluate a variable
that is already under evaluation, the second evaluation will hang.
The reason is
that GHC knows the variable is under evaluation, so the new
evaluation just waits for the result before continuing, but of
course this isn't going to happen because the first evaluation is
stopped at a breakpoint. Control-C can interrupt the hung
evaluation and return to the prompt.

The most common way this can happen is when you're evaluating a
CAF (e.g. main), stop at a breakpoint, and ask for the value of the
CAF at the prompt again.

[5] Note that packages
only contain compiled code, so debugging a package requires
finding its source and loading that directly.

[6] We
originally provided bindings for all variables in scope, rather
than just
the free variables of the expression, but found that this affected
performance considerably, hence the current restriction to just the
free variables.